[ruby-core:114658] [Ruby master Bug#19857] Eval coverage is reset after each `eval`.

Issue #19857 has been updated by mame (Yusuke Endoh).


> By the way, did you realise, eval coverage is already merged, if it's a subset of the eval'ed code?

Yes, I think that that is bad. I think I missed it because I didn't review https://github.com/ruby/ruby/pull/6396 enough. I want to limit it even from now.

I think "coverage for eval" feature was a good idea for measuring the coverage of Rails view code, but I would like to design it to the minimum necessary for that purpose. Currently, I think it is too flexible than I expected.

The design I am currently considering is as follows.

* If there are loaded and eval'ed codes with the same path, only the loaded code is measured
* If there are multiple codes eval'd with the same path, the first (or the last) eval'ed code is measured

I need to take time to understand and organize the current situation and carefully consider how it should be designed. However, I am not likely to be able to spend sufficient time to the coverage library for a while, so please wait.

BTW, I'm not keen on having `Coverage.start` have a new keyword argument for this purpose. Coverage library already has many different modes. I don't want to complicate things further. (This is mostly my fault, of course.)

----------------------------------------
Bug #19857: Eval coverage is reset after each `eval`.
https://bugs.ruby-lang.org/issues/19857#change-104488

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
----------------------------------------
It seems like `eval` based coverage is reset every time eval is invoked.

```ruby
#!/usr/bin/env ruby

require 'coverage'

def measure(flag)
  c = Class.new
  c.class_eval(<<~RUBY, "foo.rb", 1)
    def foo(flag)
      if flag
        puts "foo"
      else
        puts "bar"
      end
    end
  RUBY
  
  return c.new.foo(flag)
end

Coverage.start(lines: true, eval: true)

# Depending on the order of these two operations, different coverage is calculated, because the evaluation of the code is considered different, even if the content/path is the same.
measure(false)
measure(true)

p Coverage.result
```

Further investigation is required.



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